fix(ToC): fix mobile styles (#4636)

### Related Ticket(s)

#2545 

### Description

This fixes an issue where the section navigation dropdown is incorrectly positioned, allowing contents of the page to be seen behind it when scrolling.

### Changelog

**Changed**

- top positioning of section navigation dropdown

**Removed**

- {{removed thing}}

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<!-- *** "package: services": Services -->
<!-- *** "package: utilities": Utilities -->
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packages/styles/scss/components/dotcom-shell/_dotcom-shell.scss

@@ -19,7 +19,7 @@
     padding-top: $carbon--layout-04;
 
     .#{$prefix}--tableofcontents__sidebar {
-      top: 49px;
+      top: 48px;
     }
   }